A variable capacitive element is a component used in an electrical circuit, such as a variable frequency oscillator, a tuned amplifier, a phase shifter, and an impedance matching circuit. In recent years, an increasing number of variable capacitive elements have been mounted in a portable device.
A variable capacitive element produced by using the MEMS techniques can realize a high Q value with small energy loss as compared to a varactor diode principally used as a variable capacitive element at present. Therefore, the variable capacitive element produced by using the MEMS techniques is expected to be developed and put to practical use.
Hereinafter, a variable capacitive element produced by using the MEMS techniques is referred to as a “variable capacitive element” simply.
In general, a variable capacitive element is so structured that the capacitance is varied by adjusting a distance between two opposed electrodes.
FIGS. 1A and 1B are cross-sectional views illustrating an example of a structure of a conventional general variable capacitive element 10.
Referring to the variable capacitive element 10 of FIG. 1A, a fixed electrode 12, a movable electrode 13, a dielectric layer 14 for covering the fixed electrode 12, a pair of support layers 15a and 15b for supporting the movable electrode 13, and the like are provided on a substrate 11. A void is provided between the movable electrode 13 and the dielectric layer 14. A driving voltage V may be applied to the fixed electrode 12. The movable electrode 13 is connected to the ground through the support layers 15a and 15b. 
In the case where no potential difference is applied between the fixed electrode 12 and the movable electrode 13, the movable electrode 13 is separated from the fixed electrode 12 (see FIG. 1A). In this state, if a driving voltage V is applied to the fixed electrode 12 to apply a potential difference between the fixed electrode 12 and the movable electrode 13, an electrostatic attractive force generated therebetween attracts the movable electrode 13 closer to the fixed electrode 12, so that the distance therebetween is shortened. If the potential difference is equal to or greater than a predetermined value, the movable electrode 13 is in contact with the fixed electrode 12 through the dielectric layer 14 (see FIG. 1B).
FIG. 2 is a diagram illustrating an example of a relationship between a driving voltage V and a capacitance C, i.e., a C-V characteristic, in the variable capacitive element 10.
Referring to FIG. 2, while a positive driving voltage V is applied and the value thereof is increased, the capacitance C exhibits little change from the minimum capacitance CS in the beginning, but rapidly increases when the value of the driving voltage V is VI1 or close thereto, and then, the capacitance C exhibits little change from the maximum capacitance CL. Thereafter, when the value of the driving voltage V is reduced from this state, the capacitance C exhibits little change from the maximum capacitance CL for a while, but rapidly drops when the value of the driving voltage V is VO1 or close thereto, and then, the capacitance C exhibits little change from the minimum capacitance CS. Likewise, when a negative driving voltage V is applied and the value thereof is reduced, the capacitance C exhibits little change from the minimum capacitance CS for a while, but rapidly increases when the value of the driving voltage V is VI2 or close thereto, and then, the capacitance C exhibits little change from the maximum capacitance CL. Thereafter, when the value of the driving voltage V is increased from this state, the capacitance C exhibits little change from the maximum capacitance CL for a while, but rapidly drops when the value of the driving voltage is VO2 or close thereto, and then, the capacitance C exhibits little change from the minimum capacitance CS. As long as the capacitance C exhibits little change from the minimum capacitance CS or the maximum capacitance CL, the capacitance C is regarded as being constant at the value of the minimum capacitance CS or the maximum capacitance CL.
The capacitance C becomes the smallest value (the minimum capacitance CS) for a case where the movable electrode 13 is farthest from the fixed electrode 12, i.e., the case of FIG. 1A. The capacitance C becomes the largest value (the maximum capacitance CL) for a case where the movable electrode 13 is in contact with the fixed electrode 12 through the dielectric layer 14, i.e., the case of FIG. 1B.
In the meantime, the variation path of the capacitance C with respect to the driving voltage V is different between the case where the capacitance C increases and the case where the capacitance C drops. In short, the voltage VI1 is not equal to the voltage V01. The same is similarly applied to the voltage VI2 and the voltage VO2. It is known that the variation in the capacitance C against the driving voltage V exhibits so-called hysteresis.
In the case where the variable capacitive element 10 is used digitally, the driving voltage V is so controlled that the capacitance C of the variable capacitive element 10 takes either the minimum capacitance CS or the maximum capacitance CL. Referring to FIG. 2, for example, if the capacitance C is to be set at the minimum capacitance CS, the driving voltage V is set at a voltage VOFF (=zero). If the capacitance C is to be set at the maximum capacitance CL, the driving voltage V is set at a voltage VON1 or a voltage VON2.
In the case of changing the capacitance C, continuous application of the driving voltage V having the same polarity causes positive or negative charges to be accumulated in the dielectric layer 14. It is known that the charges are accumulated in this way.
FIGS. 3A and 3B are diagrams illustrating an example of a relationship between a driving voltage V and a capacitance C, i.e., a C-V characteristic, when charges are accumulated in the variable capacitive element 10.
When charges are accumulated in the dielectric layer 14, the movement of the movable electrode 13 is influenced by the electrostatic force due to the charges. Thus, the C-V characteristic of the variable capacitive element 10 exhibits a characteristic different from that under the state where no charges are accumulated in the dielectric layer 14. For example, FIG. 3A illustrates a state in which positive charges are accumulated in the dielectric layer 14. In this case, the C-V characteristic is shifted toward the negative driving voltage V as compared to the state in which no charges is accumulated. Further, FIG. 3B illustrates a state in which negative charges are accumulated in the dielectric layer 14. In this case, the C-V characteristic is shifted toward the positive driving voltage V as compared to the state in which no charges is accumulated.
In such cases, even if a driving voltage V that can inherently assign the capacitance C to the minimum capacitance CS or the maximum capacitance CL is applied, the value of the capacitance C does not change to the minimum capacitance CS or the maximum capacitance CL in some cases. In the illustrated examples of FIGS. 3A and 3B, even if the driving voltage V is set at the voltage VOFF (=zero), the value of the capacitance C sometimes does not change to an intended minimum capacitance CS. Thus, it is impossible to operate the variable capacitive element 10 in a stable manner, which is a problem.
In order to prevent a voltage characteristic from varying due to the charge in an insulation film, there is proposed a device in which the shape of the insulation film is improved to control the amount of the charge therein (Japanese Laid-open Patent Publication No. 2003-136496). However, it is difficult to use a semiconductor production method to form an insulation film with the shape disclosed in Japanese Laid-open Patent Publication No. 2003-136496.
To cope with this, a driving method called bipolar driving is proposed in which the polarity of a driving voltage V to be applied is turned from one to the other at predetermined time intervals to suppress the shift of the C-V characteristic.
There is also proposed a mirror control device in which a driving voltage to be applied to an electrode is an alternating voltage in order to suppress the occurrence of drift of a mirror (Japanese Laid-open Patent Publication No. 2008-052270).
FIG. 4 is a diagram illustrating an example of a time series variation of a driving voltage V and a capacitance C for a case where the variable capacitive element 10 is driven in a bipolar manner.
Referring to FIG. 4, in the bipolar driving, when the driving voltage V is applied in order to keep the capacitance C at the maximum capacitance CL, the driving voltage V is applied in such a manner that a positive voltage VON1 and a negative voltage VON2 are alternately applied at relatively short time intervals.
In the case where a driving voltage V having one polarity is applied for a long period of time, or, alternatively, in the case where a large difference is found between a period of time during which a driving voltage V having one polarity is applied and a period of time during which a driving voltage V having the other polarity is applied, charges are more likely to be accumulated in the insulation layer 14. The bipolar driving, thus, is effective to suppress the shift of the C-V characteristic.
As illustrated in FIG. 4, however, in the case of the bipolar driving, the capacitance C becomes lower than the maximum capacitance CL at a time when the polarity of the driving voltage V is turned from one to the other. In short, the bipolar driving is disadvantageous in that the capacity of the capacitance C varies. For this reason, the polarity of the driving voltage V is changed from one to the other only at a time when the capacity variation does not affect the operation of the device. This limits the cases in which the variable capacitive element 10 is driven in a bipolar manner.